1. Field of the Invention
The present invention relates to medical devices, and more particularly to stent-based venous valves.
2. Discussion of the Related Art
The vertebrate circulatory system comprises three major types of blood vessels; namely, arteries, capillaries and veins. Arteries carry oxygen-rich blood from the heart to the other organs and veins carry oxygen-depleted blood from the organs back to the heart. The pulmonary vein is an exception in that it carries oxygen-rich blood from the lungs to the heart. When an artery enters an organ, it divides into a multiplicity of smaller branches called arterioles. Metarterioles are small vessels that link arterioles to venules, which are the multiplicity of smaller vessels that branch from veins. Capillaries branch off from and are connected to metarterioles. Capillaries also interconnect with one another forming long and intricate capillary networks. After blood supplied by arteries courses through an organ via a capillary network, blood enters the venules which eventually merge into veins and is transported back to the heart.
Given the nature of the circulatory system, it is easy to understand that blood pressure in arteries is much greater than in veins. To compensate for the much lower blood pressure, veins comprise low flow resistance tissues and venous valves. The primary benefit of venous valves is their ability to limit the backflow of blood traveling through the venous portion of the circulatory system. Numerous venous valves are located throughout the veins, thereby ensuring that the blood travels through the veins and towards the heart.
The normally low blood pressure in the venous portion of the circulatory system is supplemented by the contraction of skeletal muscles. Essentially, the contraction of the muscles compresses and drives the blood through the veins. The venous valves check the backflow of blood through the veins, thereby ensuring that blood is driven back to the heart. The backflow checking function performed by the venous valves also minimizes the effect of a sudden increase in blood pressure caused, for example, by heavy exertion. In addition, venous valves also evenly distribute blood in the veins by segregating portions of blood flowing through the venous portion of the circulatory system. Any damage to the venous valves disrupts the normal flow of blood.
Venous valves are particularly important in the lower extremities. The venous system in the lower extremities generally consists of deep veins and superficial veins, which lie just below the skin surface. The deep and superficial veins are interconnected by perforating veins. Blood generally flows upwards through the legs towards the heart and from the superficial to deep veins. The venous valves are situated in the deep, superficial and perforating veins to ensure the normal direction of blood flow.
Venous valves can become incompetent or damaged by disease, for example, phlebitis, injury or the result of an inherited malformation. Incompetent or damaged venous valves usually leak blood. The backflow of blood passing through leaking venous valves may cause numerous problems. As described above, blood normally flows upwards from the lower extremities, and from the superficial to deep veins. Leaking venous valves allow for blood regurgitation reflux causing blood to improperly flow back down through the veins. Blood can then stagnate in sections of certain veins, and in particular, the veins in the lower extremities. This stagnation of blood raises blood pressure and dilates the veins and venous valves. The dilation of one vein may in turn disrupt the proper functioning of other venous valves in a cascading manner. The dilation of these valves may lead to chronic venous insufficiency. Chronic venous insufficiency is a severe form of venous disease and is a pathological condition of the skin and subcutaneous tissues that results from venous hypertension and prolonged stasis of venous blood due to valvular incompetence both of a primary nature and of a secondary nature following past illnesses of the venous subsystem. Chronic venous insufficiency progresses through various stages of symptom severity which in order of severity include venous flare, edema, hyper-pigmentation i.e. discoloration of the skin, eczema, induration i.e. thickening of the skin, and ulcers. If neglected, chronic valve insufficiency may necessitate amputation of the neglected limb.
Numerous therapies have been advanced to treat symptoms and to correct incompetent valves. Less invasive procedures include compression, elevation and wound care. Compression involves the use of elastic stockings to compress the affected area. Compression is a conservative therapy and is typically effective in a majority of cases. However, the elastic stockings are uncomfortable and expensive. Continuous elevation is frequently used to treat venous ulcers. Elevation of the affected limb improves venous return, reduces the discomfort of ulcers, and encourages healing. Elevation, however, is contraindicated in patients with cardiopulmonary insufficiency. Wound care involves the use of antibiotics and antiseptics. Topical antibiotics and antiseptics are frequently utilized to treat ulcers. Zinc paste bandages have been a primary dressing for over a century. However, these treatments tend to be somewhat expensive and are not curative. Other procedures involve surgical intervention to repair, reconstruct or replace the incompetent or damaged venous valves.
Surgical procedures for incompetent or damaged venous valves include valvuloplasty, transplantation, and transposition of veins. Valvuloplasty involves the surgical reconstruction of the valve. Essentially, valvuloplasty is a procedure to surgically modify the venous valves to xe2x80x9ctightenxe2x80x9d them. Transposition of veins involves surgically bypassing sections of veins possessing the incompetent or damaged valves with veins possessing viable valves. Transplantation involves surgically transplanting one or more of a patient""s viable valves for the incompetent or damaged valve. A more detailed discussion of these surgical procedures is given in xe2x80x9cReconstruction of Venous Valvesxe2x80x9d, R. Gottlub and R. Moy, Venous Valves, 1986, Part V, section 3.
The above-described surgical procedures provide somewhat limited results. The leaflets of venous valves are generally thin, and once the valve becomes incompetent or destroyed, any repair provides only marginal relief. Venous valves may also be damaged when the valve is being reconstructed, transpositioned, or transplanted. The endothelium tissue layer of the vein may also be damaged during handling. This reduces the viability of the vein graft after implant. Another disadvantage with transplantation procedures is the need to use the patient""s own vein segment in order to avoid the complications posed by rejection. In addition, the use of a patient""s own vein segment predisposes that the incompetence or damage did not arise from inherited factors or diseases which will affect the transplanted valve.
Another surgical procedure involves the removal of the valve. In this procedure, the incompetent or damaged valve is completely removed. While this procedure removes any potential impediment to normal blood flow, it does not solve the backflow problem.
As an alternative to surgical intervention, drug therapy to correct venous valvular incompetence has been utilized. Currently, however, there are no effective drug therapies available.
Other means and methods for treating and/or correcting damaged or incompetent valves include utilizing xenograft valve transplantation (monocusp bovine pericardium), prosthetic/bioprosthetic heart valves and vascular grafts, and artificial venous valves. The use of xenograft valve transplantation is still in the experimental stages. In addition, after a given amount of time, it has been found that luminal deposits of fibrous material develops. Prosthetic heart valves are usually made from porcine valves and porcine heart valves have a geometry unsuitable as a replacement for venous valves. These types of valves are also generally larger than venous valves, and include valve leaflets generally thicker and stiffer than the leaflets of venous valves. The thicker heart valve leaflets require a greater opening pressure. The greater required opening pressure makes such valves unsuitable for the venous system. Artificial venous valves are known in the art. For example, U.S. Pat. No. 5,358,518 to Camilli discloses an artificial venous valve. The device comprises a hollow elongated support and a plate mounted therein. The plate is moveably mounted such that when in a first position, blood flows through the valve and when in a second position, blood cannot flow through the valve. A pressure differential drives the plate. Although the device is made from biocompatible materials, the use of non-physiological materials in this type of pivoting plate arrangement increases the risk of hemolysis and/or thrombosis.
The stent-based venous valve of the present invention provides a means for overcoming the difficulties associated with the treatments and devices as briefly described above.
In accordance with one aspect, the present invention is directed to an artificial venous valve. The artificial venous valve comprises a stent formed from a lattice of interconnected elements and having a substantially cylindrical configuration with first and second open ends. One or more of the elements are deformed inwardly out of the circumferential plane. The artificial venous valve also comprises a biocompatible material attached to the one or more elements thereby forming one or more valve flaps.
In accordance with another aspect, the present invention is directed to an artificial venous valve. The artificial venous valve comprises a self-expanding stent formed from a lattice of interconnected elements and having a substantially cylindrical configuration with first and second open ends and a compressed diameter for insertion into a vessel and an expanded diameter for deployment into the vessel. The one or more of the elements are deformed out of the circumferential plane at a first angle when the self-expanding stent is at its compressed diameter and at a second angle when the self-expanding stent is at its expanded diameter. The second angle is greater than the first angle. The artificial venous valve also comprises a biocompatible material attached to the one or more elements thereby forming one or more valve flaps.
The stent-based venous valve of the present invention utilizes a modified self-expanding stent to create an effective artificial venous valve. One or more elements comprising the framework of the self-expanding stent are deformed out of the circumferential plane and towards the center of the stent and a lightweight, biocompatible fabric is attached thereto. The attachment of the fabric to the elements creates flaps which function to regulate the flow of blood in the veins into which it is positioned. The slightly higher blood pressure upstream of the stent easily opens the flaps and allows the blood to flow through. In the absence of a pressure differential, the flaps return to their normally closed position, thereby substantially preventing the backflow of blood.
The stent-based venous valve of the present invention may be percutaneously delivered to the venous sub-system by releasing it from a catheter to assist or replace deteriorating natural venous valves by allowing flow towards the heart and preventing backflow. Since the venous valve is percutaneously delivered, the whole procedure is minimally invasive. The stent-based venous valve creates very little resistance in the vessel and offers minimal complication risks. In addition, since the stent-based venous valve utilizes modified existing technology, physicians will be more comfortable performing the valve replacement procedure.
The stent-based venous valve of the present invention may be more cost effectively manufactured by utilizing existing manufacturing techniques that are currently used for the manufacture of stents with only slight modification. Accordingly, high quality, reliable venous valves may be easily manufactured at relatively low cost.